With the advanced reading and recording speed of an optical disc drive, how to maintain satisfactory reading and recording quality under high reading and recording speed becomes an issue for further research and development. One of the prominent factors affecting the reading and recording quality of an optical disc drive is the focusing performance of the laser pickup head. Since whether the laser of an optical disc drive can be precisely focused on an optical disc and whether the laser reflected by the optical disc can be well received by the optical disc drive highly correlate to the distance between the laser pickup head of the optical disc drive and the optical disc, it is required to keep a well-defined focusing distance for assuring of satisfactory reading and recording quality.
Please refer to FIG. 1A, which schematically shows the distribution of signal receiving zones of an optical pickup head. The optical pickup head 1 includes a main portion 11 consisting of four signal receiving zones A, B, C and D, a first side portion 12 consisting of two signal receiving zones E and F and a second side portion 13 consisting of further two signal receiving zones G and H. When the laser emitted by the optical pickup head 1 is reflected back to the optical pickup head 1 by an optical disc (not shown), the reflected signal is received by the eight signal-receiving zones A˜H to generate eight elementary signals, and the eight elementary signals are further processed into an output signal. When the focus f of the optical pickup head 1 precisely falls on the optical disc 2, i.e. in an in-focus state as shown in FIG. 1B, the laser intensity reaching the optical disc 2 will be maximized and the reflected signal is also optimally received by the eight signal-receiving zones A˜H. On the other hand, if the optical pickup head 1 is in an out-of-focus state as exemplified in FIG. 1C, the laser power cannot be completely transferred to the optical disc 2 so that both the laser intensity received and reflected by the optical disc 2 becomes insufficient. Accordingly, it is disadvantageous for decoding signals, and thus the signal quality is adversely affected.
Therefore, for assuring of satisfactory signal quality, it is preferred that a focus-balance calibration operation is performed to adjust the distance between the optical pickup head 1 and the optical disc 2 according to practical situations. In general, the optical pickup head 1 is first moved to a preset position, e.g. position P1 of FIG. 1C, to read data stored in the optical disc 2 by processing the laser signal reflected by the optical disc 2 into an RRF or jutter signal. Then, the optical pickup head 1 is up/down moved for a couple of times to obtain more RRF or jutter signals. By analyzing the RRF or jutter signals, an optimal position P2 of the optical pickup head 1 can be located as shown in FIG. 1B. Since the signal processed by the optical pickup head 1 is and RF-type signal, e.g. an EFM (eight-to-fourteen modulation) encoded signal and there are data pre-stored in the optical disc, the focus-balance calibration operation can be readily performed either by directly observing the signal intensities of the reflected signals or detecting the jitters or errors between signals. For example, the signal intensities of the reflected signals can be realized by comparing the peak-to-peak values of the signals.
However, when the optical disc 2 is a blank disc, the optical pickup head 1 cannot generate any EFM encoded signal because of no data being read. Accordingly, no focus-balance calibration operation can be performed for the blank disc. As is understood by those skilled in the art, the reading and recording quality cannot be confirmed, for example before any data being burned into the blank disc, particularly for the optical disc drive having high reading and recording speed.